The invention relates to a method of testing a display device comprising an air-tight envelope and at least a glass part which forms part of said air-tight envelope during the manufacture thereof.
Display devices of the type mentioned in the opening paragraph are used, inter alia, in television receivers and computer monitors.
A display device of the type mentioned in the opening paragraph is known. The known display device comprises an air-tight envelope with a display window. In the case of a cathode ray tube (CRT), the envelope also comprises a cone portion and a neck which accommodates an electron gun for generating (one or more) electron beams. These electron beams are focused on a phosphor layer on the inner surface of the display window. In the case of a plasma display panel (PDP), the air-tight envelope comprises a faceplate, which serves as the display window, and a rear plate, said plates being connected to each other by means of connecting parts. A plasma display panel contains an ionizable gas in which a plasma discharge is generated, and electroluminescent or photo-luminescent phosphors being used to produce an image.
The known display device has a number of shortcomings, in particular the occurrence of product failure during the manufacture of the display device, which product failure is caused by fracture as a result of, for example, implosion of the display device during the evacuation of the envelope.
It is an object of the invention to provide a method of selecting the glass parts mentioned in the opening paragraph in an early stage of the manufacturing process of the display device, so that the risk that the above-mentioned problem occurs is reduced.
To achieve this, the method in accordance with the invention is characterized in that the glass part is warmed up at a first temperature during a first time period, whereafter the glass part is immersed in a fluid at a second temperature during a second time period, said second temperature being lower than the first temperature. The fluid may be a gas or, preferably, a liquid.
Since glass is a brittle material, it is sensitive to surface damage and stress-related phenomena. Surface damage is generally difficult to detect by people who are not skilled in the art, and adverse effects of (surface) stresses in glass may not give rise to problems until late in the manufacturing process. In addition, it is not clear how and which surface damage as well as which types of stress in the part contribute substantially to product failure during the further assembly of the air-tight envelope and the display device. Product failure is caused, in particular, by implosion of the envelope of the display device when this is evacuated (for the first time). In said evacuation process the envelope is also brought to a relatively high temperature (300-400xc2x0 C.). Such implosions are often initiated by said surface damage or too high a surface stress. When the air-tight envelope of the display device is evacuated for the first time, the display device already is in an advanced stage of assembly, so that an implosion during evacuation and warm-up implies a loss of production.
By subjecting the glass part to a thermoshock test in accordance with the invention, any defects, such as surface defects and stresses at the surface and in the interior of the glass part become visible. The method in accordance with the invention enables said surface damage and stresses to be detected at an early stage, so that such parts can be excluded from the further manufacturing process of the display device. If, for example, in the case of a cathode ray tube, a display window is subjected to the method in accordance with the invention, it can be determined, before the display window is provided with a phosphor pattern and a shadow mask, and before the display window is fritted to the cone portion of the envelope of the display device, whether surface damage on or stresses in the display window will lead to product failure at a later stage of the manufacturing process (for example during evacuation of the envelope). A fluid which can particularly suitably be used for immersing the glass part is the liquid medium water.
Factors involved in the initiation of surface damage of and stresses in glass parts of display devices are, in particular, scratches made in the manufacture of the glass parts and during positioning and handling the parts on a conveyor belt. Another important factor, in particular, for display windows of CRTs having a raised edge via which the display window is connected to the cone portion, and which edge is generally provided with connecting points for connecting a selection electrode or shadow mask, is the degree of compressive stress present in the raised edge of the display window. In general, the method in accordance with the invention does not make a distinction between surface damage and (internal) stresses of the glass part. The resistance to quenching generally is a combination of surface roughness and internal stress of the glass part. The term xe2x80x9cquenchingxe2x80x9d of the glass part is to be taken to mean, in this application, a thermal shock caused by suddenly cooling the part (xe2x80x9cthermoshock treatmentxe2x80x9d), for example by immersing in water.
Said thermoshock treatment in accordance with the method of the invention causes cracks to grow at the outside surface of the glass part. These cracks are generally caused by surface damage or they develop in a region where the stress is relatively high. Quenching of the glass part causes the outside surface to be subject to tensile stress, while the material in the interior of the glass part is subject to compressive stress; as a result, cracks do not grow through the glass (i.e. cracks do not propagate in the interior of the glass). This has the advantage that no portions of the part become detached or severed, which would lead to contamination of the set-up for carrying out the method.
A preferred embodiment of the method in accordance with the invention is characterized in that the temperature difference between the first and the second temperature ranges between 25xc2x0 and 85xc2x0, and is preferably approximately 50xc2x0.
An important criterion for a good selection test is that the method yields a reliable distinction between usable and non-usable glass parts. A xe2x80x9cnon-usablexe2x80x9d part is to be taken to mean, in this application, that there is a relatively great risk that such a part, which forms part of the air-tight envelope of a display device, will be subject to implosion during evacuation and warm-up of the envelope; conversely, a xe2x80x9cusablexe2x80x9d part runs a relatively small risk of implosion during evacuation and warm-up. In addition, care must be taken that, in the long run, the method does not adversely affect the glass part, for example, because the treatment causes the quality of the part to deteriorate, which may not give rise to problems until later in the life of the display device. If the temperature difference between the first and the second temperature is too large, i.e. T2xe2x88x92T1 greater than 85xc2x0, the risk of crack growth as a result of the thermoshock treatment is increased, which leads to a relatively high failure percentage of the glass parts, which is undesirable. In general, the failure probability increases substantially with temperature. If the temperature difference between the first and the second temperature is too small, i.e. T2xe2x88x92T1 less than 25xc2x0, crack growth occurs only exceptionally, so that the selection treatment has (almost) no power of discernment. Experiments have shown that, between said differences in temperature (25xc2x0xe2x89xa6T2xe2x88x92T1xe2x89xa685xc2x0), a noticeably different response to the thermoshock treatment occurs. Experiments have further shown that the method in accordance with the invention has a great power of discernment as to the further processability of the part at a temperature difference between the first and the second temperature of approximately 50xc2x0 (T2xe2x88x92T1≈50xc2x0).
A suitable value for the first temperature ranges between 50 and 100xc2x0 C., and is preferably approximately 65xc2x0 C. In the case of a temperature difference of, preferably, approximately 50xc2x0 (T2xe2x88x92T1≈50xc2x0), this results in a value for the second temperature of approximately 15xc2x0 C. (T2≈15xc2x0 C.).
A preferred embodiment of the method in accordance with the invention is characterized in that the glass part is a display window or a cone portion of a display device. Particularly surface damage in combination with stresses in the raised edge of the display window or surface damage in the cone portion cause undesirable product failure. A display window or cone portion which cracks as a result of the thermoshock test can be added without further treatment (as so-called cullet) to the glass mixture in the melting furnace from which display windows or cone portions are made. If the display window is already provided with a phosphor pattern and/or, during removing the frit connection between the display window and the cone portion, residues of materials (phosphor, cone glass or fritted glass) remain in or on the display window, the composition of the glass mixture in the melting furnace is adversely affected.
A preferred embodiment of the method in accordance with the invention is characterized in that the fluid comprises a liquid having a coefficient of thermal conduction (xcex) above 0.4 Wmxe2x88x921Kxe2x88x921. A liquid having a relatively high coefficient of thermal conduction allows an effective heat transfer of the second temperature to the glass part, if said part originates from an environment having a higher first temperature. The higher the coefficient of thermal conduction, the more effective the thermoshock treatment is. Water is a particularly suitable liquid.
Preferably, the fluid comprises a liquid such that the product of the specific mass (xcfx81) and the specific heat (cp) is greater than xcfx81xc3x97cp=2xc3x97106 Jmxe2x88x923Kxe2x88x921. Water is a particularly suitable liquid.
By using the method in accordance with the invention, the risk of fracture or implosion of the display device during the manufacture of the display device is reduced, which has a favorable effect on the reduction of the failure percentage and hence on the cost price.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.